1. Field of the Invention
The present invention relates to an ionization type smoke detector.
2. Description of the Related Art
A conventional ionization type smoke detector comprises a smoke detecting section having an inner ionization chamber defined between an inner electrode and an intermediate electrode and functioning as a reference resistance element, and an outer ionization chamber defined between the intermediate electrode and an outer electrode; a sensor output section for detecting a voltage change across the outer ionization chamber of the smoke detecting section and outputting the voltage change as a sensor output; a fire discriminating circuit for producing a fire discrimination output when the sensor output reaches a fire discrimination level; and a fire signal transmitting section for transmitting a fire signal in response to the fire discrimination output, thereby carrying out fire monitoring. Also, a constant-voltage circuit is provided in the conventional ionization type smoke detector to ensure stable fire monitoring even if the detector is connected to any of various fire receivers having different supply voltages.
A sensitivity of the conventional ionization type smoke detector is adjusted as follows. (1) In the case where fire discrimination is effected upon the turning-on of a MOS (Metal Oxid Semiconductor) type field effect transistor with its gate connected to the intermediate electrode, the resistance value of a resistor which is connected to a source of the MOS type field effect transistor is adjusted. (2) In the case where fire discrimination is effected by a comparator, the resistance value of a voltage dividing resistor which applies a reference voltage for the fire discrimination to the comparator is adjusted.
In either of the above sensitivity adjusting methods, however, the adjustment has been troublesome because of the necessity of selecting a different resistance value for each of the detectors. Further, in the conventional detectors, the voltage at which the MOS type field effect transistor turns on or the voltage at which the comparator produces the fire discrimination output, i.e., the sensor output obtained from the outer ionization chamber varies with each detector. Accordingly, in order to check how far the sensor output under a smoke-free condition in use has deviated from an initial value, the initial value must be subtracted from the current sensor output for each detector, which has been laborious procedure.
Furthermore, the constant-voltage circuit of the conventional ionization type smoke detector comprises a transistor, a Zener diode connected to a base of the transistor, and a resistor connected between a collector and the base of the transistor.
However, when a high voltage is supplied from a fire receiver, the current flowing through the Zener diode of the constant-voltage circuit becomes larger than in the case of a low supply voltage being supplied. Accordingly, the number of detectors connectable to the fire receiver must should be restricted, or it is necessary to increase the capacity of a battery power supply which is provided for backup in case of a main power failure.